For example, JP-A-57-142003 (Patent Document 1) discloses the following antennas. That is, it discloses a monopole antenna in which a flat-plate type radiation element 3001 having a disc shape is erected vertically to an earth plate or the ground 3002 as shown in FIGS. 45A and 45B. This monopole antenna is designed so that a high-frequency power source 3004 and the radiation element 3001 are connected to each other through a power feeder 3003 and the height of the top portion of the radiation element 3001 is set to a quarter wavelength. Furthermore, it also discloses a monopole antenna in which a flat-plate type radiation element 3005 whose upper peripheral edge portion has a shape extending along a predetermined parabola is erected vertically to an earth plate or the ground 3002 as shown in FIGS. 45C and 45D. Still furthermore, it discloses a dipole antenna in which two radiation elements 3001 of the monopole antenna shown in FIGS. 45A and 45B are symmetrically arranged as shown in FIG. 45E. Still furthermore, it discloses a dipole antenna in which two radiation elements 3005 of the monopole antenna shown in FIGS. 45C and 45D are symmetrically arranged as shown in FIG. 45F.
In addition, JP-A-55-4109 (Patent Document 2) discloses the following antennas, for example. That is, a sheet-type elliptical antenna 3006 is erected vertically to a refection surface 3007 so that the major axis thereof is parallel to the reflection surface 3007, and power supply is carried out through a coaxial power feeder 3008, as shown in FIG. 45G. Moreover, FIG. 45H shows an example where the antenna is configured as a dipole. In the case of the dipole type, the sheet-type elliptical antennas 3006a are arranged on the same plane so that the minor axes thereof are located on the same line, and a slight gap is disposed so that a balanced feeder 3009 is connected to both the antennas.
Besides, a monopole antenna as shown in FIG. 45J is disclosed in “B-77: BROADBAND CHARACTERISTICS OF SEMI-CIRCULAR ANTENNA COMBINED WITH LINEAR ELEMENT”, Taisuke Ihara, Makoto Kijima and Koichi Tsunekawa, pp77 General Convention of The Institute of Electronics, Information and Communication Engineers, 1996 (hereinafter referred to as “non-patent document 1”). As shown in FIG. 45J, a semicircular element 3010 is erected vertically to an earth plate 3011, and the nearest point of the arc of the element 3010 to the earth plate 3011 serves as a feed portion 3012. The non-patent document 1 shows that the frequency fL at which the radius of the circle almost corresponds to a quarter wavelength is the lower limit. Furthermore, it also describes an example where an element 3013 achieved by forming a cut-out portion in the element 3010 shown in FIG. 45J is erected vertically to the earth plate 3011 as shown in FIG. 45K, and that little difference exists in VSWR (Voltage Standing Wave Ratio) characteristic between the monopole antenna shown in FIG. 45J and the monopole antenna shown in FIG. 45K. Furthermore, it also discloses an example where an element 3014, which is formed by connecting an element 3014a, which resonates at fL or less and has a meander monopole structure, to an element with the cut-out portion as shown in FIG. 45K, is erected vertically to the earth plate 3011 as shown in FIG. 45L. Incidentally, the element 3014a is disposed to be accommodated in the cut-out portion. Incidentally, in connection with the non-patent document 1, disc type monopole antennas are described in “B-131 IMPROVED INPUT IMPEDANCE OF CIRCULAR DISC MONOPOLE ANTENNA”, Satoshi Honda, Yuken Ito, Hajime Seki and Yoshio Jinbo, 2-131, SPRING NATIONAL CONVENTION of The Institute of Electronics, Information and Communication Engineers, 1992 (hereinafter referred to as “non-patent document 2”), and “WIDEBAND MONOPOLE ANTENNA OF CIRCULAR DISC”, Satoshi Honda, Yuken Ito, Yoshio Jinbo and Hajime Seiki, Vol. 15, No. 59, pp. 25–30, Oct. 24, 1991 in “TECHNICAL REPORTS OF THE INSTITUTE OF TELEVISION” (hereinafter referred to as “non-patent document 3”).
The antennas described above pertain to a monopole antenna in which a flat-plate conductor having various shapes is erected vertically to the ground surface, and a symmetric dipole antenna using two flat-plate conductors having the same shape.
Besides, U.S. Pat. No. 6,351,246 (Patent Document 3) discloses a symmetric dipole antenna having a special shape as shown in FIG. 46. That is, a ground element 3103 is provided between conductive balance elements 3101 and 3102, and terminals 3104 and 3105, which are lowest portions of the balance element 3101 and 3102, are connected to the coaxial cables 3106 and 3107. Negative step voltage is supplied to the balance element 3101 via the coaxial cable 3106 and terminal 3104. On the other hand, positive step voltage is supplied to the balance element 3102 via the coaxial cable 3107 and terminal 3105. In this antenna 3100, though the distance between the ground element 3103 and the balance element 3101 or 3102 is gradually increased from the terminal 3104 or 3105 toward the outside, it is necessary to input different signals as described above to the balance elements 3101 and 3102, and in order to obtain desired characteristics, it is necessary to always use three elements, that is, the balance element 3101 and 3102 and the ground element 3103.
In addition, FIG. 47 shows a glass antenna device for an automobile telephone disclosed in JP-A-8-213820 (Patent document 4). In FIG. 47, a fan-shaped radiation pattern 3203 and a rectangular ground pattern 3204 are formed on a window glass 3202, a feed point A is connected to the core wire 3205a of a coaxial cable 3205, and a ground point B is connected to the outer conductor 3205b of the coaxial cable 3205. In this Patent document 4, the shape of the radiation pattern 3203 may be an isosceles triangular shape or a polygonal shape. Moreover, the shape of the radiation pattern 3203 may be a shape in which a shape similar to the fan shape, the isosceles triangular shape or the polygonal shape is respectively removed from the inside thereof. Furthermore, there is a description that the rectangle may be removed from the inside of the ground pattern 3204.
Furthermore, US-A-2002-122010A1 (Patent Document 5) discloses an antenna 3300 in which a tapered clearance area 3303 and a driven element 3302 whose feed point 3305 is connected to a transmission line 3304 are provided within a ground element 3301 as shown in FIG. 48. Incidentally, the gap between the ground element 3301 and the driven element 3302 is largest at the opposite side to the feed point 3305 on the driven element 3302, and the gap therebetween is smallest in the neighborhood of the feed point 3305. The driven element 3302 is equipped with a concavity at the opposite side to the feed point 3305 of the driven element 3302. The concavity itself is opposite to the ground element 3301, and it serves as means for adjusting the gap between the driven element 3302 and the ground element 3301. Incidentally, it discloses a shape without any concavities.
Besides, JP-A-2001-203521 (Patent document 6) discloses a microstrip patch antenna 3400 as shown in FIG. 49. The microstrip patch antenna 3400 is such that a ground plane 3404, a microstrip patch 3402, and a triangular pad (feed conductor) 3403 connected to the microstrip patch 3402 are formed of conductive metal on a dielectric substrate 3401. Incidentally, the microstrip patch 3402 is fed from a feed point 3405 through the triangular pad 3403 as a feed conductor. Although not shown, from the operation principle of the microstrip antenna, the microstrip patch antenna 3400 as shown in FIG. 49 is not suitably operated unless the ground is disposed opposite to the dielectric substrate 3401. Besides, since the area of the ground plane 3404 is very small, it is not conceivable that the ground plane functions as a radiant element. Further, in the microstrip antenna, a current flowing in the radiation conductor is not a direct radiation source, and in FIG. 49, a current flowing in the triangular pad 3403 and the microstrip patch 3402 does not serve as a direct radiation source. Besides, a reception frequency bandwidth of the microstrip patch antenna 3400 disclosed in the patent document 6 is as narrow as 200 MHz with respect to the center frequency of 1.8 GHz, the triangular pad 3403 does not function as the radiation conductor, and it is conceivable that the microstrip patch 3402 is a radiation conductor of a single frequency (1.8 GHz). As stated above, the microstrip patch antenna 3400 shown in FIG. 49 is a microstrip antenna and is not a monopole antenna in which a current flowing in the radiation conductor contributes to radiation. Besides, it is not a traveling-wave antenna in which the wide bandwidth is realized by continuously changing a current path flowing in a radiation conductor. Further, since the reception frequency bandwidth is single, it is not a dual band antenna.
Thus, although there are various antennas, the size of the conventional vertical mount type monopole antenna becomes large. In addition, vertically erecting the radiation conductor against the ground surface makes control of the distance between the radiation conductor and the ground surface difficult, and accordingly makes control of the antenna characteristics difficult. Furthermore, as for the conventional symmetric dipole antenna, because the two radiation conductors having the same shape are used, it is difficult to control the distance between the radiation conductors and to control the antenna characteristics. Still furthermore, as described above, even if a cut-out portion is provided for the radiation conductor of the vertical mount type monopole antenna, the improvement of the VSWR characteristic is not achieved. In addition, although the antenna shown in FIG. 45L resonates at frequencies lower than fL because of the element 3014a, and multiple resonances are achieved, the VSWR characteristic at frequencies lower than fL is poor, and the antenna characteristics presently required for the dual band antenna are not realized. Incidentally, in the patent documents 1 and 2, and non-patent documents 1 to 3, there is no description and suggestion for working the shape of the ground surface.
Besides, the special symmetric dipole antenna described in the patent document 3 has a problem on the implementation, in which a lot of elements and two kinds of signals, which are supplied to the elements, must be prepared. In addition, the ground pattern 3103 is opposite to the balance element 3101 and 3102, but the sides of the ground element 3103, which are opposite to the balance element 3101 and 3102, are straight lines. On the other hand, a side portion of the balance elements 3101 and 3102, which are opposite to the ground element 3103, is almost straight, too. Accordingly, the change of the distance between the ground element 3103 and the balance element 3101 or 3102 is straight.
In addition, in the glass antenna device for the automobile telephone in the patent document 4, the distance between the radiation pattern and the ground pattern straightly changes. Because the adjustment of the distance cannot be carried without change of the angle of the fan, the fine adjustment is impossible. Furthermore, although there is a description for removing the inside of the ground pattern, there is no disclosure as to processing an external form of the ground pattern to adjust the distance with the radiation pattern. Moreover, there is no disclosure for providing a cut-out.
In addition, though the antenna described in the patent document 5 aims at miniaturization, the structure that the driven element is provided within the ground element cannot achieve the sufficient miniaturization. Furthermore, if the driven element is surrounded by the ground element, the space between the ground element and the driven element should be large because the coupling between the ground element and the driven element becomes too strong. This prevents from the miniaturization of the antenna. Incidentally, the shape of the ground element does not have a tapered shape with respect to the driven element.
Further, with respect to the microstrip antenna disclosed in the patent document 6, although the shape appears to be such that both the triangular pad and the microstrip patch contribute to radiation, the triangular pad does not serve as the radiation conductor, but is merely the feed conductor. Thus, this antenna is the antenna in which the reception frequency bandwidth is single, and is not the dual band antenna.    Patent document 1    JP-A-57-142003    Patent document 2    JP-A-55-4109    Patent document 3    U.S. Pat. No. 6,351,246    Patent document 4    JP-A-8-213820    Patent document 5    USPA2002-1220101A1    Patent document 6    JP-A-2001-203521    Non-patent document 1    “B-77: BROADBAND CHARACTERISTICS OF SEMI-CIRCULAR ANTENNA COMBINED WITH LINEAR ELEMENT”, Taisuke Ihara, Makoto Kijima and Koichi Tsunekawa, pp77 General Convention of The Institute of Electronics, Information and Communication Engineers, 1996    Non-patent document 2    “B-131 IMPROVED INPUT IMPEDANCE OF CIRCULAR DISC MONOPOLE ANTENNA”, Satoshi Honda, Yuken Ito, Hajime Seki and Yoshio Jinbo, 2-131, SPRING NATIONAL CONVENTION of The Institute of Electronics, Information and Communication Engineers, 1992    Non-patent document 3    “WIDEBAND MONOPOLE ANTENNA OF CIRCULAR DISC”, Satoshi Honda, Yuken Ito, Yoshio Jinbo and Hajime Seiki, Vol. 15, No. 59, pp. 25–30, Oct. 24, 1991 in “TECHNICAL REPORTS OF THE INSTITUTE OF TELEVISION”